1. Field of the Invention
The present invention relates to a directional coupler, an antenna device, and a transmitting-receiving device which are useful for a radar or the like with which the distance to and the relative velocity of a detection object are measured by transmission-reception of an electromagnetic wave, for example, in the millimeter wave band.
2. Description of the Related Art
In recent years, a so called "millimeter wave radar for car-mounting" has been developed, of which the purpose lies in that the distance to and the relative velocity of a vehicle running ahead or behind are measured in a vehicle running on a road and so forth. In general, the transmitting-receiving device of the millimeter wave radar of the above type includes a module comprising a millimeter wave oscillator, a circulator, a directional coupler, a mixer, an antenna, and so forth which are integrated together, and is attached to the front or rear of the vehicle.
For example, with the module of this type, the relative distance and the relative velocity of a vehicle running ahead are measured at a vehicle running behind, by transmission-reception of a millimeter wave according to the FM-CW system or the like. The transmitting-receiving device and the antenna of the module are attached to the front of the vehicle, and a signal processing device is disposed in an optional location of the vehicle. In the signal processing section of the signal processing device, the distance to and the relative velocity of the vehicle running ahead are extracted as numerical information. In the control-alarm section, based on the velocity of the vehicle running behind and the distance between the vehicles, an alarm is given, for example, when predetermined conditions are satisfied, or when the relative velocity for the vehicle running ahead exceeds a predetermined threshold.
In the millimetric radar of the above type, the directivity of the antenna is fixed. Therefore, there may occur the case that the desired detection or measurement can not be performed depending on conditions, as described below. More particularly, for example, if vehicles run in plural traffic lanes, it can not be determined immediately whether a vehicle running ahead is present in the same lane where the vehicle is running behind, only by receiving an electromagnetic wave reflected from the vehicle running ahead. More particularly, when an electromagnetic wave is sent as a radiation beam from the vehicle running behind, a reflected wave from the vehicle running ahead, and moreover, a reflected wave from a vehicle running in the opposite lane may be received. The relative velocity determined based on the reflected wave from the vehicle running in the opposite lane is unduly high. As a result, inconveniently, an error alarm is given. Further, if vehicles are running on a curved road, a vehicle running ahead is out of the detection range of the radiation beam and can not be detected, by sending forward an electromagnetic wave as a radiation beam from the vehicle running behind, Further, if vehicles are running on a hilly road, a vehicle running ahead in the lane where the vehicle is running behind is out of the detection range of the radiation beam, and can not be detected.
Accordingly, it is speculated that the above-described problems can be dissipated by varying the direction of the radiation beam.
For example, in the case that vehicles run in several traffic lanes, two detection objects adjacent to each other in the forward angular directions can be separately detected by changing the radiation beam, operational processing, and comparing the measurement results in the respective beam directions. If the vehicles are running on a curved road, the curve of the road is decided based on the steering operation (steering by a steering wheel) or by analyzing the image information obtained with a camera photographing the forward view, and the radiation beam is directed to the direction in dependence on the decision, so that the vehicle running ahead can be detected. Further, if the vehicle is running on a hilly road, the undulation of the road is decided by analysis of image information obtained with a camera photographing the forward view. The radiation beam is directed upwardly in dependence on the decision, so that the vehicle running ahead can be detected.
However, referring to the method of changing the directivity of an electromagnetic wave in the conventional transmitting-receiving device operative in the microwave band or millimeter wave band, the whole of a casing containing the transmitting-receiving device including the antenna is rotated only with a motor or the like to change (tilt) the direction of the radiation beam. Accordingly, the whole of the device is large in size, and it is difficult to scan with the radiation beam with the direction of the radiation beam changed at a high speed.
Conventionally, by another method, beam-scan antennas for switching plural antennas to scan with a beam are employed. However, by the beam scan antenna method, it is necessary to provide as many antennas as beams. Accordingly, if the beam scan antenna is used in the transmitting-receiving device, there is caused the problem that the whole size of the device is large. Further, since as many antennas as beams are used, it is needed to arrange the respective antennas in consideration of their scan ranges. Thus the arrangement of the antennas is difficult. Further, in order to switch the plural antennas for inputting or outputting, electronic switches such as diodes or the like are used. The loss at the switching is too large to be neglected in the millimeter wave band. Further, it is needed to switch on-off the beams from the plural antennas, and therefore, it is necessary to provide as many electronic switches as antennas. The electronic switch such as a diode or the like is expensive. Thus, there is the problem that the beam scan antenna using many electronic switches costs a great deal.
In recent years, investigation on three dimensional beam scanning by which upper, lower, right, and left sections are scanned has been made. If a method of moving the whole casing of the transmitting-receiving device only by means of a motor or the like is employed, there is caused the problem that the whole structure is further enlarged, and the scanning at high speed is difficult.
Further, for three dimensional beam scanning by means of a multi-beam antenna, it is needed to arrange antennas in the upper, lower, right, and left sections. Thus, there is caused the problem that the whole structure is large in size, and the connection, switching, and arrangement of the respective antennas is very difficult.